do you use it as a foliar spray? What is in the soil mix?

I have the same q's as Randy, what's the soil mix?

If it's mostly composted wood as most soil mixes are, there's not much there for the plants.. Fish and Kelp are still only going to provide so much micro-nutrients.

Maybe consider adding some well aged home made compost to your mix.

The 5-1-1 is much higher in nitrogen than the 2-4-0. I prefer using a more balanced fertilizer, since my soil usually don't need lots of any one thing.

Phosphrous may be lacking and the 2-4-0 will help more than 5-1-1. If it's been colder, the phosphorus doesn't get absorbed as easily and the plant can turn purplish.

We used to use the used fish tank water and dilute it when we had gold fish.

My soil recipe for most seeds is
peat
vermiculite
lime
perlite
Homemade compost.

So I do try to sweeten it up, but never have taken a professional PH test

You can't stress brassicas from cold around here because it is too mild even in winter. The soil in the cells gets much hotter than out in the garden because of the lack of a heat sink.

I have to water cell flat trays every day, sometimes twice a day in my dry climate, maybe the compost nutrients from the compost are leaching? But then they are in trays so where would it go? Hmm.

Sometimes I bottom water with the fish and kelp and sometimes pour from the top. More often than not from the top I guess.

Get a pH test, even the cheap kind will help on that point. Knowing your pH is very important for growing different plants.

since you need to water every day, you will need to fertilize more often. It's not that the nutrients are leaking, but they are being used up. I usually fertilize at 1/2 strength every other watering.

I keep forgetting how warm you are Rustico, while I'm sitting with almost freezing temps this morning AGAIN.

I still guess that your phosphorous is low, and would change to the 2-4-0 next time you buy.

Water from bottom will help with the 'natural' salts from accumulating on the top of the soil (that white crusty stuff).

i think its the heat stress. maybe shading them for part of the day would keep the soil cooler? can you direct seed them?

I bet you are right about the heat , Randy, I can and should shade them....was thinking about that for fall when it is really hot and dry, but I guess in spring it is necessary too. Lots of difficulties with direct seeding of most types of vegetables.